Some known cordless telephone systems employ a radio frequency (RF) carrier detect system in which the base unit checks for the presence of an RF carrier during the period when the cordless telephone unit is in the inactive "on-hook" state. These known systems use a hardware analog carrier detect circuit.
A microcomputer-based RF carrier detection arrangement is disclosed in U.S. patent application Ser. No. 08/171,353 (Fossaceca, et al. ) filed Dec. 22, 1993, and assigned to the same assignee. In that application it was pointed out that, unfortunately, during the manufacturing process, an adjustment procedure must be performed on the hardware analog carrier detect circuitry in every cordless telephone set in order to ensure correct triggering at the specified thresholds. This adjustment procedure tends to be difficult, time consuming, and often causes the detection of the RF carrier to be unreliable.
Moreover, since the carrier detect circuitry is in the base unit of the cordless telephone set, these systems are only capable of detecting interference at, or near, the handset transmitter frequencies (i.e., the base unit receiver frequencies). Interference occurring at, or near, base unit transmitter frequencies will go undetected, and may lead to disrupted communication between the base unit and the handset.
One might think that RF carrier detect circuitry should also be installed in the handset to alleviate this problem, but two separate obstacles have heretofore prevented such a simple solution. First, another hardware analog RF carrier detect circuit had to be added to the handset, increasing the cost of manufacturing, further complicating the design, and increasing manufacturing assembly time as well as the calibration time of each unit. In addition, the handset is a free-standing unit, and as such, exhibits a poor ground-reference, which adds to the difficulties of adjusting the RF carrier detector threshold potentiometer reliably.
Second, cordless telephone handsets are intended to be portable, and consequently are battery-operated. Ideally, one would like the handset to draw no power at all in the inactive mode. Unfortunately, the receiver in the handset must be energized, and thus drawing power, in order to receive incoming calls via transmission from the base unit. In order to conserve power, a common solution to this problem is to repetitively "pulse" the handset "on" and "off" in a short duty cycle to look for incoming RF transmissions from the base unit. The duty cycle for the handset is typically 40-60 milliseconds (ms) "on" and several hundred milliseconds "off". Not all of the "on" portion of the duty cycle is available for use, however, because 15-20 ms is required for the PLL and microprocessor circuitry to stabilize after being "powered-up", and up to an additional 5 ms is required for the hardware analog RF carrier detect circuitry to stabilize. In the worst case, the additional 5 ms stabilization time would unacceptably dissipate 25% of the available active monitoring period of the handset.